text:
Why do ELT panel switches have batteries in them?
Does anybody know why ELT remotes (the panel mounted switches with LEDs) have batteries in them?
The ELT itself has a big battery and there's a wire between them that the ELT could use to supply power to the remote.
So - why do ELT remotes have their own, separate, battery (in a place that's often hard to access for replacement)?
Is it some kind of regulatory requirement? Why?

Welcome to aviation.SE! I reworded your question a little because I wasn't sure what an "ELT remote" meant. If I changed it too much then you can always roll back my change or edit it again yourself.

The battery in the remote switch like the ACK 406 ELTs powers the LED in the remote (the ACK ELT has a separate battery for its alarm buzzer).  On some ELTs the remote's battery may also power the alarm buzzer (the one that warns you the ELT is broadcasting in case you missed the red LED) that you install near the remote (my old AmeriKing 406, now a desk paperweight on account of AmeriKing's TSO certification fiasco, had the same remote that ACK uses and it appeared to power the both the LED in the remote and the little alarm buzzer, because the alarm was wired directly into the remote with no other apparent source of power).
Some units like the ARTEX ME406 don't have a battery for the remote switch (the ARTEX unit's buzzer has its own battery), and on those units you have to either connect a remote switch power source wire to the airplane's electrical system, or if in a no-electrics airplane, you have to cobble together a separate battery power supply to power the remote's LED.
I couldn't find an engineering specification with the information, but I dimly recall that all this is because the ELT's internal battery can't have a potential load device, like a light or speaker (buzzer) connected, that could drain the internal battery if the external light/buzzer shorts out, so they require their own power sources.


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Is flutter speed always smaller than divergence speed for aircraft?
In aeroelasticity, there are three main phenomena that one should take care of: divergence, aileron reversal and flutter. Each of them has an associated speed at which the phenomenon might start to occur.
During wind-tunnel tests it is possible to increase the flutter speed to have access to the divergence speed first by using some small masses smartly placed on the wing. This is due to the fact that usually flutter speed is smaller than divergence speed.
Is it always the case for aircraft (without additional masses on the wing)? If not do you have any example? If yes do you have an explanation why or a mathematical proof of it?

Wow, this is a really tricky question. It needsca lot of knowledge about aircraft design I honestly don't have and what you won't learn during normal flight training. Anyway I hope you'll find a good answer to this question.

Flutter speed will usually be smaller than divergence speed. Flutter will appear first because as a phenomenon, by definition, describes the state of the deformation of the wing where it deflects for example up and down in a vibrating fashion. Divergence on the other hand is when the moments from the aerodynamic forces acting on the wing are larger than the restoring moments due to structural stiffness thus resulting in failure.
For the sake of simplicity -since the entire calculation is pretty lengthy- let's imagine an airfoil at an angle of incidence θο that's been twisted by an angle θ due to aerodynamic loading. This additional angle θ is used to measure the twisting strain (dynamic) energy 0.5*K*θ^2 where K is the stiffness of the airfoil - much like a spring's K constant. An increase in speed will increase angle θ and the corresponding strain energy stored in the structure. When the speed reaches the divergence speed the restoring forces are smaller than the aerodynamic forces and the angle θ becomes infinite but since infinite is impossible in reality, the structure fails.
Very early airplanes (Wright, Langley) had major divergence problems and where unable to fly although they had airfoils capable of generating the lift necessary. The same airplanes with stiffer wings in later designs where able to fly. Modern airplanes due to proper design and stiffer materials have divergence speeds higher than flutter speeds. Divergence is a measure of the overall stiffness of the structure and is also used in order for the aircraft to get certified.

Welcome to aviation.SE and thanks for this nice answer!

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text:
Is there anything extra required to spin a Cessna 172 in the Utility category?
I'm trying to find a suitable aircraft to get my spin endorsement for my CFI rating but am running into trouble with most aerobatic aircraft due to my height (too tall).
My last thought is that I could use a 172 in the utility category (I already know I fit in one and can have full control authority - I can get my legs under the dash to reach the pedals and my knees don't block the yoke).
It looks like 14 CFR 91.307(d) says I wouldn't need a parachute since I'm doing this with a flight instructor for training for a certificate.
In Section 2 of the 1978 Cessna 172N POH under the heading Utility Category (page 2-7), it says

This airplane is not designed for purely aerobatic flight. However,
  in the acquisition of various certificates such as commercial pilot
  and flight instructor, certain maneuvers are required by the FAA. All
  of these maneuvers are permitted in this airplane when operated in the
  utility category.

Provided I am operating the aircraft in the Utility category (CG within limits and Weight 2000 lbs. or less), is there any reason I couldn't use a Cessna 172 to obtain my spin endorsement?
(I know that the gyro in the Attitude Indicator can tumble - I'm not sure how to get around that if I can't cage it - but is it really harmful? I've heard this is one of those perpetual aviation myths - that it doesn't really cause damage, it just takes time to reset)

I can see where a Decathlon or a Pitts might be a tight fit, but what about an Extra?  They're pretty roomy.

@DanPichelman I was told that people 6'4" often have a hard time fitting in the Extra (closing the canopy) - I'm 6'9".

You're 6'9" and you can fit into a CE-172 as well?

@CarloFelicione yep, as long as the seat goes down and up! Also, you get to really know your CFI :)  (Although the 182 is much more comfortable). Also, no problem seeing over the nose on climb-out!

Don’t know if I’m too late but I too had the same issue with height and weight. I was recommended to do my spin training in a Stearman. 300lbs+ and 6ft tall, I accepted. I found an aerobatic school called “biplane fun” in Compton California to receive my endorsement.

You and the instructor and how much fuel? (to make the weight limit).  The CG may be more important (utility is more forward).  Expert advice from an aviation school that flies them is in order.

Provided you meet ALL of the requirements for utility category operation as spelled out in the POH (within W&B limits for the utility envelope, no aft passengers, empty baggage compartment, and anything else you may find in the Limitations and W&B sections of your POH) a Cessna 172 is approved for intentional spins, and may be used for spin training.
As to the gyro instruments, spinning MAY cause the gyro instruments to tumble. This is not good for the instrument (it beats up the bearings - do it too much and you'll wind up with an early gyro overhaul).
Newer vacuum gyros tend to be more resilient than the old ones that birthed the "Don't let the gyros tumble" rule of thumb - they'll tolerate more extreme angles without tumbling - so there's less of a need to worry about damaging them in spin training.
(Solid-State gyros obviously won't suffer damage as they have no gimbals and bearings to beat up. They may however become "confused" and go offline if they sense something they're not designed for.)

As a note, I would not do spin training in a Cessna 172 (or any other typical trainer where spins are only authorized within the utility envelope) that hasn't had a recent physical weighing, unless your operating weight and balance range is squarely in the middle of the utility envelope.
A plane with many years of "computed" W&B updates may in fact weigh 100lbs more (or, rarely, less) than the weight and balance sheets indicate, and the empty center of gravity could vary by an inch or more.
If you are going to be performing intentional spins it behooves you to ensure that you're actually operating within the envelope for which recovery has been tested.

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Could winds of up to 150 km/h impact the structural loads on a Boeing 777?
From 15 October 2016, AI 173 from Delhi to San Francisco started flying over Pacific. Jet streams over Pacific helped Air India in reducing the flight time by 2 hours, despite increasing the travel distance by 1100 km. This also made AI 173 the world's longest flight, with total distance of over 9300 miles (15,000 kilometres). It completes the journey in 14.5 hours, with average speed of 1040 km/h; 560 knots.
(Adapted from here.)
As many people are wondering, could winds of up to 150 km/h impact the structural loads on the B777-200LR? Is it ever likely to be dangerous?

150 km/h (8,000 fpm) vertical wind may be a problem while 300 km/h horizontal tail wind jet streams are usually welcome, but not always.

Wow! I wonder if pilots can actually "feel" that they are in a jetstream without having to compare speed relative to the ground vs air speed.

@Nav: You cannot sense speed (only acceleration), especially when above clouds at the same velocity. Note that Earth is spinning very fast, but we don't feel it.

High wind does not overload aircraft for the same reason that the rotation of the Earth does not overload road vehicles.

Note that SIN-EWR still holds the record for the world's longest scheduled flight (direct route would be 9,534 sm, actual would be more.) This might be the longest currently-active scheduled flight by actual flown distance, though. By direct distance, the DXB and DOH to AKL routes are longer, though. SIN-SFO is also longer than DEL-SFO by direct distance.

@reirab They are longer by Great circle distance. But by actual distance travelled, this is now longer than Emirates Dubai to Auckland. This flight now flies opposite to great circle route. SIN-EWR might change that though..

@reirab here's the even better thing. On 21st October, AI 173 flew over 10,000 miles. This leaves even SIN-EWR behind. Flightaware

@anshabhi Yeah, all flights vary route (and, therefore, distance flown) from one day to the next depending on weather, traffic, and such. That's why direct (great circle) distance is normally used instead when "longest scheduled flight" is being discussed.

No, there's no considerations on structural load because the airplane is moving in relation to the air mass. Think of the jet stream as a river, as air is just another fluid. A boat heading downstream on a fast moving river will move faster in relation to the land, but in relation to the water it's moving the same speed as if it were on a lake with no current.

ymb1's and GdD's answers are very good, I'd like to expand them a bit further.
The loads on the airplane structure are caused by forces between air and the plane.
If the stream can be described as laminar flow, there is no difference between flying in headwind, tailwind or calm air. Except for ground speed, obviously. When onboard and near the wing, you won't notice anything at all.
When the stream is (highly) turbulent, that means there are significant changes in the wind directions, pressure, temperature and density these inhomogenities creates additional loads to the structure. You will notice this; the fight will be like rollercoaster ride and you will see the wings flapping and wild corrections.
This characteristic can be estimated using Reynolds number, where stream velocity is used, but it is not the only variable to form the criterion.
tl;dr
It depends.

What do you want to compute by the Reynolds number here? It is in millions whatever wind you have. What conclusion would you like to take out of that? The turbulence in the air itself is governed by other processes and the Reynolds number scaled by the airplane's dimensions doesn't play any role at all.

@VladimirF I don't want to estimate Re near the airplane, it is obvious that it will be high around (behind) it. I want to estimate the jet stream flow behaviour without the plane (several meters in front of it). Turbulences are governed by the air flow, the "weather" - humidity, temperature, termics. I just wanted to show, that the air speed is not the lonely factor, and Re came to me as a first rough estimate.

I can't how that would work. Maybe the Richardson number, but I simply can't see a useful direct application of the Reynolds number there.

The Reynolds numer tells you if molecular viscosity is important or not. In the jetstream you can forget the viscosity. It is completely negligible and therefore so is the Reynolds number. You must use criteria that describe the relevant physics in your situation.

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text:
How can civil ATC maintain safety with military aircraft around?
I've been asking myself for a while how ATC can maintain air traffic safety with regard to military aircraft - or if they just don't.
As I understand it, all bigger planes (let's for now focus on commercial airliners) have mandatory transponders and warning systems like TCAS and similar. By usage of such systems, ATC knows their position, altitude, heading etc. which is used for planning of take-offs and landings just as well as air traffic safety. Also, systems like TCAS allow pilots to directly respond to unsafe situations even if ATC "didn't see it coming". In short, lots of regulations and techniques to make flying safe.
Now I know even less about military aircraft, but I do know they have transponders, too. I suppose, since enemies would have it a bit easy, those transponders don't operate the same way civil transponders do; or at least they use different frequencies, encryption and what-not. I don't know if they have TCAS but, I suppose, even that could possibly be exploited by enemies.
If I assume correctly, civil ATC cannot necessarily see military aircraft. I've read in another question that FAA doesn't have much power over military but that's just the US anyways. Also, the answer in yet another question suggests that military aircraft do not have to maintain separation.
So, what's the verdict? Do military aircraft assume civil regulations while not in "combat mode"? Or are civil ATC indeed oblivious of military air movements and the military has to provide safety by regulating military and watching civil traffic?

Military aircraft do have transponders that can reply to civil ATC radar and TCAS interrogations. Normally military aircraft operating in civil airspace are visible to civilian ATC and also will trigger TCAS advisories and alerts if they are getting close to airliners.
During  wartime operations, and sometimes during combat practice in dedicated airspace, the transponder is operated in a different mode and will not be replying to civil radar interrogations.
Typically when operating in civil airspace, military aircraft will fly under the civil regulations for that airspace. Training flights are usually conducted in dedicated airspace under military traffic control.
In some cases the military ATC assumes responsibility over military aircraft in otherwise civil ATC controlled airspace. In such a case the military ATC will be responsible for the separation between the military aircraft themselves and between military and civilian aircraft.
In other cases military aircraft may be allowed to maintain their separation from each other on their own, without civil ATC interfering in their operations. This allows for operations where military aircraft flying close to each other like formation flying and aerial refuelling outside the dedicated military airspace. In the USA these operations are called MARSA (Military Assumes Responsibility for Separation of Aircraft). Civil ATC is responsible for keeping Instrument Flight Rules (IFR) aircraft separated from MARSA operations. Visual Flight Rules (VFR) pilots have to maintain separation visually from MARSA flights and vice-versa. If VFR pilots are in contact with ATC they will be advised of ongoing military operations.

@DeltaLima Nice, thank you! Am I right assuming that military aircraft, even with deactivated transponders, should still be visible to civil ATC on primary radar? Or can they actually hide completely these days?

If ATC has primary radar, which is often the case,  they can see military aircraft.  Unless they are   'stealth' aircraft.

Military aircraft do flight plan and even if they switch-off transponder, ATC knows  that some flights are operating in their airspace. In that case ATC will not allow civil flights to operate in that area or they will increase the separation in addition to traffic information service . during war time operations the airspace is normally restricted for civil flights.

@ShahabMohamed Indeed. Usually when military aircraft are operating exclusively in an area, temporary flight restrictions are put in place the airspace effectively making it a military training zone. Civil aircraft are not allowed to plan their routes through restricted areas.  There is no increased separation, there is just a no-go area.

As an additional note, when operating in combat conditions, military aircraft will turn off anything that might give away their position to the enemy - transponders, nav lights, etc. I have heard combat aircraft scream overhead (on their way to bombing missions), invisible to the naked eye, only to hear them, then see them leisurely make their return trips with nav lights on. It's eerie.

Another good example of TCAS at work keeping military and civil aircraft apart: http://avherald.com/h?article=4414b03b&opt=0

@UnrecognizedFallingObject Thanks for that link, I don't know whether to laugh or cry reading that incident report. Unbelievable that the FAB doesn't switch their TCAS replies off during an intercept.

U.S. military aircraft have multi-mode transponders. within U.S. airspace, they "squawk" using Mode 3, which is identical in its form and operation to civilian Mode C (a four-digit code plus altitude, allowing ATC systems to calculate true distance instead of using slant distance which varies with altitude). While in theater, fighters use enhanced transponder modes that allow for digitally-encoded and even encrypted interrogation/response to prevent false positives, as well as additional information sharing from the INS such as tail number, GPS position, bearing and airspeed, reducing the need for radio communications and radar systems between friendlies. Similar information is also available from civilian planes using Mode S transponders, which most commercial airliners possess.
Military jets would disable their transponders in the following situations, and very rarely or never in any other:

By explicit instruction of ATC. A malfunction or an incompatible squawk mode can cause conflicting data on ATC displays, and in certain circumstances even if it is operating properly ATC will tell the pilot to "stop squawk" and track the plane by other means.
When intercepting a plane with TCAS. Traffic Collision Avoidance Systems interrogate nearby aircraft to obtain direction, distance (based on round-trip time) and altitude data from their transponders which it can use to alert the pilot when an aircraft is within minimum separation distances or is on a potential collision course. A military fighter intercepting a civilian aircraft will look like it's on a collision course to the civilian plane's TCAS (it pretty much will be until the interceptor forms up off the beam of the interceptee). This can lead to the pilot of the intercepted aircraft taking evasive maneuvers recommended by TCAS to avoid the potential collision, causing a "dogfight" as the interceptor tries to form up while the interceptee constantly evades. There is a standard squawk code in the U.S. for an intercepting aircraft (0000, 7777 for military) which some TCAS systems will treat differently, but in some cases the interceptor has to at least partially disable their transponder in order to avoid triggering TCAS.
When flying in formation. In close formations such as those used during timed flybys at sporting events and airshows, only the flight leader will squawk. This is for two reasons; first, it's redundant for anyone else in the formation to do so, as the composition and layout of the formation can be established by radio contact and nearby traffic advised. Second, ATC radar resolution can be limited especially at longer ranges, so it can be confusing for the ATC's IFF system to interrogate along an azimuth and receive several simultaneous signals.

"In close formations such as those used during timed flybys at sporting events and airshows, only the flight leader will squawk."
This isn't always necessarily true. The transponder requirements vary by facility.  Most civilian ATC will require only the flight lead to remain mode C; however, we fly through several facilities that assign discrete codes to each member of the flight, regardless of how close they are to lead.

Sometimes they just can't. Basically you can turn off transponder on any airplane. In this case all ATC can rely on is the primary radar if it's good enough and they can see objects like military aircraft. The good example will be Russians flying bombers around Europe. There have been several near collisions already. Best options is to send military jet to intercept bomber and follow it. If intercepting jet has transponder on then ATC can know approximate position of the bomber.

The jet isn't mostly to show where the bomber is. It's to tell the bomber crew "Hi. We see you."

First the size of aircraft have nothing to do with transponders.
Certain airspace mandate having transponder. You cannot operate there without one. It follows that big aircraft need to fly through any airspace so they must have transponders anyways.
Military aircraft are not exempt from this rule. They at least must have the transponder.
Military aircraft have this "due regard" regulation, meaning depending on what they're doing they can either keep their transponder on and operate under ATC control or if they have their own radar and have a need to, they could turn off their transponders and do their own separation from other aircraft. This mostly happen only in time of war, when they need to intercept potentially unfriendly aircraft or under certain training ops where they need to hide themselves from other aircraft to simulate air combat.
When aircraft is just going from A to B in friendly airspace, their transponders will be turned on (I guess unless bordering unfriendly airspace). They have no justification to turn it off.
But then we have the fun stuff, like an SR71 or F22 flying above 60000ft, which is class E airspace, which does allow them to turn off their transponders and use their own radar to separate. But nowadays there are no civilian aircraft left that can go there (last one was the Concorde). The SR71 fly so high they really didn't need to worry about transponder or radar at FL800 !
Technically there are some exceptions to the must have transponder rule though, but only for uncritical airspace. You cannot go without a transponder into Class A or B airspace, period.


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Could passengers realize a possible emergency?
First, to clarify, I'm not looking for nightmare stories here. I'm not curious about the fears of passengers but rather about what's actually noticeable for them. It's pretty clear that passengers not flying too often tend to be afraid during turbulence. But, I suppose, very few of them ever experienced a stall or similar.
Now, reading about crashes like the one of AF447 where a pilot only very late in the emergency situation re-enters the cockpit, I'm curious about why that is. I've never been on a plane during a stall, let alone an airliner. Don't passengers (and thus a pilot outside the cockpit) experience a stall as something very unusual, dramatic event? As another example, the crash of 4U9525 which was, as I understand it, a very "common descent": could a passenger even notice a difference between the upcoming crash and, say, an emergency landing at a close airport that the pilot just didn't have the time to explain beforehand?
Given "weird feelings" during flights, often related to turbulence which is not at all dangerous -- can a passenger "feel" the difference between those, a controlled flight into terrain, and an imminent crash?
Update
I disagree with closing this questions as a duplicate of What things can a passenger look out for, to indicate an emergency?. I was not looking for how passengers could intercept with flight management. Let's make it more concrete with the AF447 example then:
According to Wikipedia, about half a minute after the problem of wrong speed indication occurred, the aircraft went into stall. Another three minutes later the now returned pilot realized the stall only after he was made aware of the PF pulling up the whole time. Given this example, does a stall with an A330 feel so much "normal" that even a highly experienced pilot couldn't "feel" the missing lift? Will the passengers have thought the flight was still normal at this point?

The issue with CFIT is that it is "controlled": eg, within normal operating limits.   I expect the only clue would be a passenger looking out the window and realizing they are too low relative to the expected phase of flight (cruise).

I don't know if there's a good answer: you already mentioned how difficult it would be for passengers to tell the difference between something dangerous and something that's just unusual, and as this answer points out, most passengers don't know anyway. If something really 'dramatic' is happening like fire in the cabin or structural failure then of course people would know something is going wrong, but your question seems to be mainly about stalls or other loss of control incidents, where it may be unclear what's happening, even to the crew.

There is an appreciable loss of lift (not stall) when the high lift system is retracted. Producing an effect similar to an stall but much lighter.

Can you explain more why you think this isn't a duplicate? If you want to focus on something like stalls, it would help to edit the question to make that more clear.

I wasn't out for only stalls, that's why I didn't explicitly single them out. One problem is to know what kind of flight situations there are that could lead to imminent danger. I was hoping for an answer like "A stall feels like ... which is something all passengers would realize; a CFIT is never noticeable because ...; an airliner going into a dive ..." and so on. I could look for more examples, I guess. But maybe there aren't really that many.

I think the passengers on 4U9525 may have suspected something was up when the captain was outside the cockpit trying to break down the door to get back in.

And, although many sources have said that the passengers on AF447 would not have known there was a problem, there’s no way to really know that for certain. The plane experienced roll oscillations and roll angles above 40°, and the vertical speed, as recorded on the FDR jumped around quite wildly. There would have been buffeting and the noise would have been abnormal and almost certainly noticeable. They would have been buckled in due to upcoming turbulence, but I’m not convinced it was the easy ride to the water that people suggest it was.

One of the survivors (a passenger) of Air Florida Flight 90 was a Private Pilot and

...Stiley, a pilot himself, said he realized that something was wrong as the plane headed down the runway. He said there was still snow and slush on the wings and he remembered wishing he could get off the plane.

source

On the other hand, the thousands of times someone thinks that and the plane isn't in trouble don't get reported on.

While it's certainly possible to come up with scenarios, either historical or suggested by imagination, where passengers would or could know that "something isn't right," the number of scenarios where "false positive" conclusions (taking as an emergency something that in fact, isn't) and "false negatives" (not knowing about something emergent actually going on) will probably be far greater.
All sorts of things can come up that will be fairly transparent to the passengers, until a pilot gets on the PA and announces that they'll be diverting to a more immediate landing: loss of a hydraulic system or navigation system, a medical emergency on board, some types of electrical malfunctions, etc etc.
On the other hand, many of the things that produce the most noticeable cues that "something isn't right" are simply turbulence... it doesn't feel right because it isn't particularly normal, but it's no emergency, just a rough ride.  With a LOT of experience, one might be able to tell the difference between a jolt of turbulence (or wake turbulence) and, say, the momentary yaw associated with an engine shutdown, but who has "experience" riding through multiple engine shutdowns these days?  It's too rare an event for many people to have been through more than zero, let alone two or three.
So, while some things are clearly not normal -- flames coming out of an engine, smoke in the cabin, fuel streaming out of a wing, etc, these sorts of things are really uncommon.  I once asked our safety department what was the most common emergency that they saw in all the safety reporting, and the answers were #1 diverts of an ill passenger, and then #2 issues with flaps/slats not extending or retracting correctly (either actual issues, or false indications of trouble).  And everything else was pretty far behind those two causes.  So, if you weren't close by the medical event, your chances of knowing about it (besides the PA) are slight, and unless you really know what to look for, the chance of spotting an abnormal flap/slat configuration are pretty remote as well.
So I wouldn't assume that very many aircraft emergencies have symptoms that are immediately visible to the passengers.  Some will, but many won't, and not everything that seems unusual is a sign of any looming problems.

Thank you for your answer! I understand many indicators of impending danger can not be seen without a very trained eye. But what about if the emergency is already about to be fatal (see my update on the question)?

I suppose the follow-up question is more appropriate. "What are you going to do about it?


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Why there is no wake turbulence right after touch down or before rotation?
It is said that wake turbulence occurs when an aircraft generates lift and there is no wake turbulence after touch down or before take off. That's how we avoid wake turbulence. However, I've thought we have lift while on landing roll or take off roll because there is the angle of incidence and at least a certain amount of speed, just not enough lift to be airborne. Based on my idea, I don't really understand why does wake turbulence suddenly disappear like a magic when we land.
Can someone explain this please?

There is wake turbulence on the ground; wake turbulence occurs anytime there the wings are creating lift (though turbulence occurs downwind of the aircraft anytime there is relative motion between the airframe and the air). Avoidance of wake turbulence has to do with location and strength of that turbulence.

When the aircraft is on the ground and the airfoil isn't at a positive AOA, it isn't producing lift. Usually you need to rotate to start generating lift, which is done by the elevator to put the wing in a positive AOA and start the lift generation.

@RonBeyer Many aircraft do have a positive angle of attack on the ground. See the An-2—which is notorious for wake turbulence—for an extreme example.

@JonathanWalters There are almost always cases to the contrary, however I think what I said is still true *"When the aircraft is on the ground and the airfoil isn't at a positive AOA", obviously the AN-2 doesn't fit that statement. I didn't say that all airfoils aren't at a positive AOA on the ground...

@RonBeyer Agreed

Even if you were to find yourself in wake turbulence while on the ground, the contact of the wheels on the runway will stabilise the aircraft in such a way that the wake turbulence will not be as noticeable as if you were airborne, and would affect you much less.

Well there may be vortex formation on the ground under certain conditions, but it doesn't stick around long enough to really be measured or cause trouble.
Any time a wing is producing lift it will generate a wake. This happens even if the wing generates just enough lift to make the aircraft "lighter" but not enough to get it off the ground: Vortices will still start to form at the wingtips, but as they begin their rotation (downward and outward) they will quickly hit the ground and be dispersed.
If the runway were a narrow strip with the wingtips hanging out in free air you would still see vortex formation in these edge cases, but pilots generally don't like to take off on a tightrope.
Airplanes also usually don't stay in that kind of edge case situation where the wing is generating lift but the wheels are still on the ground for very long: Either they're taking off (soon after the wing starts producing some lift it's producing enough lift and the airplane begins flying), or they're landing (within a few seconds after you touch down you've slowed to a point where the wing's lift production, and thus vortex formation, is negligible). Thus for practical purposes of wake turbulence avoidance the wake begins when the airplane rotates, and ends where it touches down.
It's also worth noting that if you happen to be following close enough for the incipient wingtip vortices to cause trouble on the ground you'll probably be experiencing other issues (jet blast, prop blast, etc.) which will remind you that it's prudent to maintain a safe separation distance from other aircraft on the ground.

Wake turbulence is related to the amount of lift produced. This in turn is proportional to the angle of attack of the wing. Before take-off rotation the wing produces little lift, so the effect of wake turbulence is small. Ground effect will weaken the wake further, but it would technically be wrong to say that no wake turbulence is produced.
The strength of the wake turbulence of a flying aircraft is proportional to the mass of the aircraft and inverse with airspeed squared. This means that the turbulence gets stronger while the aircraft descends and decelerates, and wake turbulence reaches a peak right before touchdown. With the wing close to the ground, ground effect will weaken the turbulence, so when the wheels touch the ground the intensity of the wake is maybe two thirds of its free flight strength at the same speed. Only when the aircraft derotates (the process when pitch attitude is reduced until the nose wheel touches the ground), the wake turbulence will be significantly reduced.
After derotation the lower zero-lift angle due to the flap deflection for landing will let the wing produce more lift at the same speed than in case of the aircraft with flaps set for take-off. Again, ground effect and extending spoilers or wing-mounted speed brakes reduces the wake (with flaps down the ground effect is even stronger), but here it would be even more incorrect to say that there is no wake turbulence. It is only much weaker than in flight.


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How can I avoid floating during short-field landings?
I fly a Piper Cherokee PA-28R for my training and have a question about the techniques used for a short field landing.
What is the difference between a normal landing roundout/flare and a short-field landing roundout/flare? Should I begin the roundout and/or idle the power sooner to get minimum floating? Where does the difference in 'float distance' come from?

Excessive airspeed? It's not how fast you can fly, it's how precise.

I'm not sure but minimum float and short field might not be synonymous (if I can use that word).  You can have minimum to no float at all on a huge runway and you can successfully land at a short field with the float you're used to.  Understanding the float and analyzing the way you yourself fly will help you get the landing you're hoping for.  I would have rephrased the question (don't want to edit your question) to:  what are the techniques used for a short field  landing.?.

You might want to mention what aircraft you fly: sometimes there are specific techniques for individual types.

Definitely depends on the aircraft.

It's all about energy management, you need to keep your airspeed at the low end.

The issue is an energy management issue.  Having low kinetic energy (speed) when having low potential energy (altitude) at the desired touch down point is the objective. There are numerous techniques to do so, some with less risk then others. Having a better understanding of the energy budget will help you manage things better, and your CFI(s) can help you find technique(s) which will work for you.

I did all my training in PA-28's (Warrior and Archer II mainly) and here is my advice)
First off, practice makes perfect in all cases here.
Second, I am not a CFI and you should ALWAYS seek the experience of a CFI to help you learn how to fly properly. Discuss everything with your CFI before trying anything.
The PA-28 is one of the most benign airframes and even in the bumpiest the bumpiest of conditions you can fly it by the numbers every time. Floating on landing is generally caused by (in my experience) 2 things.

Excessive speed on final - Remember you will only land nicely once you have gotten rid of all excess speed. This happens during your round-out and flare to landing. On a short field even a perfect landing will take up more runway if you are overspeed since you need to bleed that speed off.
Ballooning - This is generally a result of over speed and over flaring. Since the PA-28 is a low wing (compared to the high wing of the C172) an over-flare to early will cause significant ballooning as you effectively bounce off the ground effect. A proper timed round out and flare will come in time.

One thing you can do to mitigate this (and I found this helped as a student pilot for me) is to fly a longer final leg (and thus a longer downwind). When I was learning to fly a stable approach it took time for me to get the settings in and really nail the approach. A longer final will help you settle the plane into place as a student and really nail a maneuver like this. Generally tight patterns are taught but if its safer and better for you to fly a longer final and nail the maneuver, do it.
In the PA-28's you can (and should) be right on 65 kts when doing a soft field. Practice descending at a constant airspeed at altitude with your instructor. 500ft/min right on 65Kts with full flaps, then come back into the pattern and use what you learned to come in right as you should.
Cut the power a bit earlier. Generally you are most likely taught to cut the power over the numbers and touch down at the markers. In a short field situation you can cut the power a bit earlier, pitch down to keep you speed up, and touch down right on the numbers. You should have no issue getting the plane down in under 1000Ft.
Train at an actual short field (or as short as you are allowed to fly). My flight school generally had us going between 5000ft airports with 2 runways (we were based at KPNE). Crosswinds were never really an issue and runway length left plenty of margin for error. My instructor made it a point to take me to 3000 X 60 fields to really get practice in (the shortest the fligh schools insurance would cover). Depending on where you are training out of this may not be an issue.
One more general habit of new pilots is over correcting and getting your self into an increasingly corrective state. This happens both laterally and vertically but in your case the vertical over correction is going to be the killer. If you are comming in and notice that you are low you add power, pitch up a bit and now you are too high. You pull power and pitch back down but this builds up some speed. Now you find your self over correcting all over the place to get back to a stabilized aproach. This is where the long final will help. You need to really get the plane on that 65Kt mark, trim it off, leave the power alone and fly it in. If you get bumped by a gust or a thermal wait a second and see if the termal settles then and only then should you come in with a small correction.

The key to landing where you want is to control your airspeed on final.  Consider this, during takeoff the airplane doesn't just jump off the ground; it needs to be going at a sufficient speed to allow the airplane to climb.  The same is true in reverse while on landing.
Here is an excerpt from a C-172 POH.  Notice the normal landing speed range is between 60-70 KIAS and the short field landing is precisely 61 KIAS.  That speed will ensure little to no flare.
I always aim just a little short to allow for a little flare during the short field landing.

The answers noted above are helpful advice and I'll add the following as well.
1) It's all about airspeed control on final.  You gotta keep that needle pinned at your target airspeed while remaining on glide path.
Practice technique:  on a fair weather day, calm winds and at an airport with either a visual glideslope indicator or an ILS approach, set up on a long final and establish yourself on glideslope at proper airspeed.  Note your nose attitude, trim settings and engine tachometer or manifold pressure gauge and remember them for future reference.  Make a mental photograph of the approach attitude and use this for future reference as well.
On any approaches thereafter, as you finish turning onto final, set your power, trim and attitude to these numbers and allow the aircraft to settle onto the approach.  The aircraft should shortly establish itself at the target airspeed with neutral elevator input and little adjustments needed thereafter to fly the approach.  Once established, hold that chosen attitude and use small changes in power - walk the throttle - to correct for glideslope deviations, returning to your chosen power settings as the aircraft returns to the glide slope.
2) a gentle touchdown is not required if the aircraft is at the proper attitude in the flare and within limits.  A lot of new pilots think that 'greasing' a landing, that is making the smoothest touchdown possible is the hallmark of a good landing.  As a result they often fly shallow approaches and hold off the flare indefinitely using up extra runway in a float to kiss the main wheels on the pavement.  Short field landing should not be greasers.  This is why we fly target airspeeds on final so when we do round out, the airplane will be exhausted of energy and plop down on the mains quickly and as close to but aft of the runway threshold as possible.  It also guarantees better braking once on the ground.

I would be careful about saying "don't retract flaps": it's in the POH short-field procedure for many aircraft. If a DPE asks why you did or didn't do something, "it's the procedure in the POH" is always a good answer :-) Having said that, outside of checkrides I completely agree about experimenting to find out what works best. And FWIW, in my last checkride I did retract the flaps (per the POH) and the DPE said "we used to bust people for that years ago, but we don't now because it's in the POH".

For more on the flap thing, see: Raising the flaps right after touchdown. Good or bad?.

I've read the Cessna and Piper POHs on the subject and the amplified version of it usually goes something like "For maximum brake performance, retract the
flaps, hold the control wheel full back, and apply maximum brake
pressure without skidding the tires."  I'd have to have a chat with the test pilots who wrote that at Independence, KS or Vero Beach as it does not seem to conform with reality.  Unless someone can demonstrate why leaving the flaps down has some extra advantage to it, I cannot conclude that my deviation from that specified in the POH is incorrect.


text:
What is the registration number of the 10,000th Airbus?
Recently, Airbus delivered its 10,000th aircraft (an A350 to Singapore Airlines). What is its registration number?
I understand the registration number is specific to the airline, and has nothing to do with the manufacturer.

The answer you are looking for is: 9V-SMF.

Current registration number: 9V-SMF (source)
Let's try to add some related details.

Aircraft can have multiple ID during their life, at least a preliminary ID to be used by the manufacturer prior to delivery to the customer, and then at least a national registry ID.
Manufacturer pre-delivery ID
Airbus aircraft final assembly line for the A350 is in France at Blagnac, a small city near Toulouse, with parts coming from several countries. There are other final assembly lines, but none is for the A350.
The first test flights will therefore happen in France, well before delivery to the customer. The French registry delivers temporary IDs to Airbus, the same IDs being continuously reused on successive airframes. Airbus uses a lot of them.
In France temporary IDs start with letter W. This A350 has been registered F-WZFD until being delivered to Singapore Airlines:

The special 10,000th aircraft delivery logo on A350-900 MSN 54 with its temporary registration: F-WZFD (source)
This same number was previously used on other aircraft, e.g.this A350 c/n 9 and will be reused on next pre-production or pre-delivery aircraft.
Customer registration
Singapore Airlines then requested an ID which was delivered by the Singaporean registry. This ID is 9V-SMF:
This number was previously used for a B747 at Singapore Airlines.

What's the meaning of the 54 beneath the temporary registration? Disambiguating the various aircraft to have used that temporary registration?

@gsnedders: It's the manufacturer serial number (MSN), or  construction number (c/n). This airframe should be the 54th of the A350 series, a list can be found here.

The MSN Number is the Airbus Manufacturen No. all parts of this Aircraft are connected to the MSN 10000. This Aircraft will be a A320 from MEA in 2020.
But the MSN 10003 A321ACFwill be delivered n September 2019 is a IB03.

I'm a bit confused... You're claiming that the 10,000th aircraft will be delivered by Airbus in 2020? There are photos above (from around 2016) of the aircraft Airbus claims to be their 10,000th in posts dated 2016. Can you please provide some more details to explain what you mean, and, if possible, some references to support your claims?

Each Airbus type has a MSN series starting with 1, it sounds like you're talking about the 10,000th A320, not the 10,000th Airbus aircraft.


text:
Are emergency actions an appropriate part of a captain's briefing in a light aircraft?
It is generally considered that giving a captain's briefing when flying passengers in a light aircraft for recreational purposes is a good thing to do. I'm asking about flying people who may not be used to light aircraft, such as friends and family (as opposed to other pilots, who we often fly with).
In general, I give a few pieces of information:

What will happen on the ground
When to avoid talking (i.e., when it looks like I'm listening intently and noting something down, when we're approaching to land)
Where we're going, what will happen during the flight
What to do if they feel unwell

Then there is the bit I'm going to ask about; people are used to hearing about "emergency exits", and evacuation procedures etc. when flying on an airliner, so I usually consider it prudent to supply the same sort of information about a light aircraft emergency:

How to open the doors if jammed by damage
To get out as soon as we come to stop
To move as far away from the aircraft as possible, even if I am unable

However, I'm starting to wonder if this sort of information would be scaring the bejesus out of my inexperienced passengers. In the next couple of years I'd like to fly my kids who are still fairly young. I definitely do not want to make it a scary experience for them when the time comes.
How do other light aircraft pilots approach this conundrum? Be safe, and supply the important information while possibly scaring passengers, or avoid the "nitty gritty" of an emergency knowing that it is a rare occurrence indeed?
It strikes me that I could provide this information if the time comes to make an emergency landing, however knowing my own limitations I know that if that time does come, I will probably be too overwhelmed with aviating to be thinking clearly enough to provide the important information.
What is the "right" amount of emergency briefing?

Good question! Hopefully, by the time you're flying your kids around, your wife will have enough experience in the air with you that she can by your "flight attendant" and handle the flight safety/emergency briefings, and, should the need ever arise, the in-air, "Daddy's super busy right now, Susie, so I'm going to tell you what you need to do" announcements. I would think that teen/adult passengers should be able to handle a little bit of head's up, "Just in case something should go wrong" information pre-flight. If they can't, you may not want to take them...

Interesting question - I think it's all in the tone and the little details. You can convey the same information with a friendly "If we encounter any problems I'll....." vs "When we have an emergency...." etc

I do the same. Spot on!

@FreeMan its a good point, but I have zero chance of getting my wife in a light aircraft. Not her thing. I managed to once, there were 3 pilots and her in there.

It sounds like you are covering the safety material you need to from a regulatory and good sense approach. Trying to convey an emergency exit briefing during an emergency is not going to work, you need to give it all at the beginning. If you get an engine fire on startup you need your passengers to be ready.
I find that people appreciate that you give a briefing as it shows a bit of "professionalism" that re-assures them, rather than scaring them it shows you know what you are doing. I do cover the same material, and I usually do it slightly tongue and cheek in the similar manner as a commercial airline. For example when I talk about emergency exits I point to the doors to the left and right, then say something like "if there is an exit behind you then things really have gone wrong". That's just my personal style, there's no hard fast rules on it. If people decide not to fly with you after your briefing maybe you are going about it wrong though.
The approach I take with small children is different than that which I take with adults. The first flight in a light airplane is a very novel experience for anyone, with kids I explain what the experience will be like a lot more than adults. An adult will understand what you mean by climb usually, but saying "the airplane will lean back a bit and we will go up" makes more sense to a little kid.

+1 People know and acknowledge that place do have incidents (and accidents), good passengers want to know what to do in case of emergency. And if not, you don't want them as passengers in a GA plane.

If you're flying in the UK at least, then it's not only a good idea but required for legal flight. The PIC must brief the passengers on where to find, and how to use:

the emergency exits,
the safety belts or harnesses, and
the oxygen equipment and life jackets, if you have to carry them

The same applies under ICAO Annex 6 is your flight is international.
In particular, if your aircraft has kick-out panels, it's a good idea to emphasize those. Most passengers won't be used to them, and in the unlikely event that they're needed, you may not be able to explain at the time.
As you say, most people nowadays have some experience of flying on airliners, so they'll be ready for and expecting this. If you take your kids on commercial flights, you can (try to) draw their attention to the safety briefing there, so they'll be more ready for your briefing. That might be easier said than done, depending on their age and how nervous they are.
With kids, hopefully they'll start out excited rather than scared. (If they're more scared than excited, you might consider leaving it a bit longer before introducing them to GA flight.) In that case, I think you have to appeal to their sense of adventure. You don't have to draw attention to what you call the "nitty gritty": the most likely emergency where the passenger would need to know this information is a forced landing in a field, and even if that happens, the most likely outcome is that you all walk away safely. You can warn your kids that if something goes wrong with the aircraft, you might have to land in a field, and that would be scary, and this is what they'll need to know.
With your own kids, you've probably had similar talks with them about how to call the paramedics, what to do in a fire, how to deal with strangers, &c. You'll know by now what tone works best with them, and how to convey that what you're saying could be really important but probably won't happen at all. Make it clear that your passenger briefing is the same kind of talk, not something new and scary.


text:
How do aviation engineers ensure that the landing gear opens every time, without fail?
This is mostly with respect to large commercial airliners. I mean so many things can go wrong, then how have the engineers designed it to make it fail-safe? Or is it a very simple trick.
Also, are there any new developments in this area of landing gear, like lighter gear, or fixed gear etc.

Given the amount of landing gear incidents that happen, I think there's a flaw in your premise that it can never go wrong. Landing gear failure can and do happen.

FWIW, all (?) aircraft with retractable landing gear have emergency procedures to follow in case the gear doesn't extend. And pilots are regularly tested on how to follow them, at least in the case of those commercial pilots who have to do recurrent training.

@Pondlife That is at least true of all aircraft certificated under 14 CFR 23 and 25 (see §23.729 and §25.729)

The short answer is extensive testing and backup/manual over ride systems. In reality there is not a whole lot that can go wrong, landing gear systems are fairly simple in design.
This varies a bit by airframe but most landing gear systems are pretty much the same. Basically every aircraft out there, from a single engine Piper Retract all the way up through a 747 has some form of manual gear over ride system. You can read about that in this answer here. So basically if the main system fails (and it happens) they have some kind of manual over ride to force the gear down. Granted there can be cases where the manual over ride also does not bring the gear down but you can only be so redundant. Some gear is also held in place positively, in other words there is pressure on the system holding it up so that should the hydrolics fail you dont need the pressure to get the gear down. Often the gear is oriented so that gravity and aerodynamics can deploy it.
Fixed gear is generally not a great idea on anything bigger than a small trainer. Fixed gear adds drag at the cost of a simpler airplane. You can read up on a similar question here.
I dont think lighter gear is something they are looking into to solve the gear deploy issue. Generally speaking you want the gear to fall down in the event of a failure so a little extra weight is not the end of the world.


text:
Could airliners use a steam or electric powered catapult for take-off?
This would have the obvious benefits:

Less runway needed to take-off
Fuel savings
Faster turn around times for airlines

Could this be implemented in busy airports?

Related: http://aviation.stackexchange.com/q/24479/1289 ?

The first point "less runway" is arguable as the usual runway would be needed for landing, unless you use an arrestor device, which is likely a big challenge when carrying cargo or passengers. Also going from 0 to VR in less time means greater acceleration, what would be your limit?

A friendly note: this a perfectly reasonable question. There's no need to vote a question down just because the answer might be no way or because you think it might be naïve - that's not how this system works. If you're in any doubt about the value of the question, look at the quality of the answers below.

This is, by the way, exactly how aircraft carriers work for takeoffs.

@mins Typically, though, more runway is used for takeoff than landing, so it would reduce runway requirements a bit, even though it's completely infeasible for a lot of other reasons.

@reirab: Agreed. But one should never say infeasible: Airbus Eco-Climb. "The ultimate, albeit it very extreme, concept is to have a system that not only launches but also captures the aircraft, removing the need for landing gear."

If you wanted to save fuel used on the ground, using tugs for taxiing would make a much bigger difference (and is being worked on)

@DanieleProcida You are mostly correct.  Normally if a question is naïve, it means there are assumptions, or even assertions like "significant cost benefits" that are not correct.  Normally my strategy is to point those out, not just to downvote the question.  However,  If I see a question any reasonable person could answer, maybe with a few seconds on Google, I'll downvote it.  I consider good questions to be ones where the asker has put at least some thought and research in.

As a matter of fact, @CodyP, the hover over text for the upvote icon is "This question shows research effort; it is useful and clear", while the hover over text for the downvote icon is the inverse, ""This question does not show any research effort; it is unclear or not useful", so your assessment is spot on with SE's.

Related, but in a different field: Why don't we use catapults to get to space? on [space.se]. Full disclosure: The accepted answer there is my own.

@chrisH Delta Air Lines experimented with a variation of this - taxiing on only the left engine.  It did indeed save fuel, but maintenance costs on the right engine went through the roof.  Turns out the nice, long, low idle "warm up" of a taxi is good for the engine.  They went back to dual engine taxi and right engine maintenance levelled back out to match the left.

@ScottSEA Do you have a reference for this? It seems reasonable, but then it also seems like the kind of thing that an engine manufacturer might specify as part of their standard operating procedures.

@mins :  The Airbus Eco Climb is an interesting concept from Airbus. The answers I'm getting are really eye opening, to say the least, with excellent explanations provided along with reasoning. To others, I would say, I did some basic research, but could not get any relevant solution, hence I used this forum.

Let's see what the savings are:
A mid-sized airliner carries maybe 20% of its mass in fuel. This fuel has an energy density of 43 MJ per kg. Of that chemical energy at most 40% is converted into useable work. Heck, let's make this 25% so we are really conservative. Thus, the energy for the whole trip is $$E_{\text{trip}} = 0.2 \cdot 0.25 \cdot 43,000,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass} = 2,150,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass}$$
Now assume that this airliner saves the energy to accelerate from 0 to 150 knots by using a catapult. This energy is $$E_{\text{accel}} = \frac{v_{\text{takeoff}}^2 - v_0^2}{2} \cdot \text{mass} = 2,977.35 \,  \frac{\mathrm{m}^2}{\mathrm{s}^2} \cdot \text{mass}$$
Since I picked metric units, conversion is easy: $1 \, \mathrm{J} = 1 \, \mathrm{Ws} = 1 \, \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}$. I use $\text{mass}$ for the take-off mass so you don't think it is the unit meter. Now let's put that into proportion:
$$\frac{E_{\text{accel}}}{E_{\text{trip}}} = 0.001385$$
Using the catapult saves 0.1385% of the energy needed to fly a typical airliner trip, assuming the same efficiencies during acceleration as during the flight. If we take into account that jet engines are most efficient during cruise, let's double the fuel need for acceleration and make it 0.277%. Granted, it is more for short-range flights, but still insignificant to what is needed to move the aircraft 10 km up into the sky and then for a couple of hundred miles through the air at Mach 0.8. In terms of fuel mass, these 0.277% are taken from 20% of the take-off mass. So the fuel needed to accelerate to v$_0$ is 0,000554 times take-off mass.
To make a catapult launch feasible, you need to add some strength to the nose gear and the forward fuselage. The typical landing gear fraction of the take-off mass is about 3%, and the nose gear is 10% - 15% of that, so $m_{\text{nosegear}} = 0.00375 \cdot \text{mass}$. Relative to the nose gear mass, the fuel saving from using a catapult launch is $\frac{0.000554}{0.00375} = 0.0148$ or 15% of the nose gear mass. Thus, the reinforcements need to add less than 15% to the mass of the nose gear.
If we assume an acceleration of ½ g = 4.903 m/s², the take-off run to accelerate to 150 kts is 607 m. I expect that even this moderate acceleration (which requires a pulling force of half of the lift at take-off) would translate into much higher mass increases than those 15% of the nose gear mass.

You forgot to take into account (which would make the concept even less interesting) that a catapult capable aircraft needs to be built much stronger. So heavier wing and fuselage spars, heavier landing gear, etc. etc. End result is an aircraft that, to allow it to need less runway for taking off (runway that's needed anyway for landing) will be much heavier and thus require a lot more fuel while flying.

You forgot to factor in the fact that 99.9% (rough guestimate) of paying passengers would not like the massive acceleration of being shot off the ground by a catapult. While the lines at Cedar Point for the Top Thrill Dragster ride are long, they are not the longest at the park by any means.

@FreeMan: Who says you need to do the acceleration within a few meters? If the launch stretches over 2km, the acceleration would be just as today, and nobody would mind.

I made the leap from the "less runway" benefit to "aircraft carrier deck length runway". Maybe I took it to the extreme... ;)

Also, in addition to everything else, add in the cost to the airport to install, operate, and maintain the catapult - and also the added fees, surely, to whatever airline wanted to make use of it.

You forgot to factor in the mass reduction due to not needing the accelerate the fuel you would normally have to have carried to accelerate to the rotation point. Not that is matters a hill of beans. Bigger issue anyway would likely be the airport costs would increase and swamp the costs of the fuel saving anyway.

There are a couple of things in this answer that seem incorrect. In your first equation, energy spent for a trip is a linear function of efficiency. So the higher the efficiency the more energy we spend for the trip, and at zero efficiency we spend zero energy - that cannot be right. Second point: your computation of energy to reach $V_0$ is done at 100% efficiency. What process can do that?

@Koyovis: The first equation only shows how much useable energy can be extracted from the chemical energy in the fuel. At zero efficiency that is zero, and at 100% it is all the 43 MJ per kg, referenced to the MTOW. If you now use a quarter of the 100% as the useable part, the result is 2.15 MJ per kg of aircraft MTOW. Second point: If I have already accounted for the efficiency loss once, when calculating the energy available, I should not do so a second time when calculating the kinetic energy change for reaching v$_0$, should I?

The airliner uses the same fuel with the same fuel efficiency factor to reach $V_0$ on the ground. This is the amount saved when using the catapult. The engines accelerate air through the engines, analogous to this answer. The comparison should be made using fuel flow * time for the phases.

@Koyovis: Now better?

This site says: a B747 uses 5,700 lbs of fuel for take-off, out of 422,000 lbs. That is 1.35% of the max take-off fuel load for a long range airliner.

@Koyovis: Let's compare apples with apples. A takeoff is only complete at 1.3 v$_s$ and at 50 ft altitude. The page you cited most likely added fuel for taxi, too. Here I only assume the catapult accelerates the aircraft until it rotates (150 kts).

This is definitely an apple. Taxiing does not explain a factor 10. The catapult can accelerate to 1.3v after which the aircraft can partially zoom climb, as the clip in the Eco-climb article shows.

Don't stop at 150 knots. Run that catapult up the side of a mountain, and launch the airliner right into cruising altitude. Now how much fuel does it save?

@snips-n-snails The potential energy of lifting the plane by 10 km is equal to 443 m/s. There is a limit to the speed an airliner can be accelerated to - the maximum dynamic pressure is at about half that speed at sea level, or a quarter of the energy. And then the plane arrives at this altitude at zero speed. Not feasible, in other words.

It would be possible from an engineering point of view to design some sort of ground-assisted take-off launch mechanism for airliners, even though any of the benefits you describe would be greatly outweighed by new disadvantages, as outlined in other answers.
There's one new point though that I think is worth making. The shorter take-off roll that it would mean is not in itself an advantage, but a disadvantage.
The time spent gathering speed on the runway with the engines at full power is valuable. It's a chance to ensure that they, and the rest of the aircraft's systems, are operating correctly. If there's a failure such as a loss of power or a sudden drop in hydraulic pressure, the take-off roll is a good place for it to happen, because it gives the crew an opportunity to abort it safely.
That opportunity would be lost in an assisted take-off.

Dunno 'bout that.  At least in the past, the captain was supposed to rev up to 105% with the brakes locked to verify all those systems, then rev down, release brakes, and take off.

@CarlWitthoft I've never had that happen on any flight I've ever been on.

If given a choice, I would rather do final checks on the engines and systems while the aircraft was still at a stop on the ground, instead of in the takeoff roll.  You don't have as much ability to be cognizant of the check due to the increased workload of managing a moving aircraft.  It would be simpler to run through the final checks, then do the cat shot, similar to carrier launches.

@CarloFelicione without doubt, but we're talking here by definition about problems that become apparent during the take-off roll, rather than ones that would have been discovered during pre-flight checks. Sometimes, all the pre-flight checks don't reveal a problem that does manifest itself during the roll. It's not very often, but it does happen.

You all are missing the most obvious answer:  You could, but aside from a few young adrenaline junkies no one else would want to ride it.
The only real purpose for a catapult assisted takeoff is to provide and aircraft with a quick acceleration to Vr and beyond off of a short airfield.  As virtually all airports used for major commercial operations have runways of at least 1 mile in length or more, there is not an infrastructure crisis which would dictate a need for this.
If you did have an airfield which was so small that it required a CATO launch to get the jets airborne you also face the task of landing them in a small space as well.  This would require the field be equipped with arresting gear as well.
As pointed out above no existing airliners are designed to launch and recover using these systems so, even with the investment in a CATOBAR infrastructure for an airfield, no for-profit air carrier could use it.  And it offers virtually no fuel savings for the airlines.
And let's not forget human factors here:  if we take CATOBAR operations from military aircraft carriers as a yardstick for performance, a cat shot imposes a 2-2.5 G acceleration load upon the aircraft during the launch stroke and a 2-2.5 G deceleration during an arrested landing.  While I'm sure a twenty-something adrenaline junkie will get a thrill out if it, it will be an unpleasant experience for most people and quite dangerous for the elderly, the infirmed, pregnant women, etc.

Yes there would be fuel savings, from multiple sources:

Efficiency gain from the ground based launcher.
Weight saving from smaller engine size.

Efficiency gain. According to this site, a B747 uses 5,700 lbs for the take-off, out of 422,000 lbs max. fuel. That is 1.35% of fuel for a long range airliner, percentage for a shorter range airliner would be higher. The catapult or electric tow line would now have to deliver the take-off energy - if powered by electricity, the efficiency is much higher. A factor of over 2 is gained from not accelerating air but the aircraft itself, and combined cycle power is much more efficient than a single gas turbine. From wikipedia:

By combining these multiple streams of work upon a single mechanical shaft turning an electric generator, the overall net efficiency of the system may be increased by 50–60%. That is, from an overall efficiency of say 34% (in a single cycle) to possibly an overall efficiency of 62.22% (in a mechanical combination of two cycles) in net Carnot thermodynamic efficiency. This can be done because heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). In an ordinary (non combined cycle) heat engine the remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted.

So in an aeroplane, air is accelerated by a thermodynamic process with 35% efficiency. When launched by a catapult, the aircraft is accelerated from a thermodynamic process with over 60% efficiency. Total efficiency gain is the factor 2 reported above, times 60/35 = 3.4 times higher. Leading to 5,700/3.4 = 1,700 lbs required for catapult launch take-off. A potential saving of 4,000 lbs at every take-off of a B747. Of course, a lot of this is negated by having the engines running at RPMs that allow for a climb right after take-off, but even a saving of 1,000 lbs/heavy aeroplane would provide an incredible yearly saving at a busy airport like O'Hare.
Weight savings. Airbus is making a case for this (now behind an authorisation wall). Maximum thrust is only used at take-off, an assisted take-off would mean lighter engines with associated lower fuel burn. From the article:

Listen to the changing sound of engines during flight and it’s obvious: an aircraft draws on its power reserves more during takeoff than at any other time. The power needed to take off is determined based on a number of factors - including runway length, wind speed, temperature, and the weight of the aircraft itself.
However, this takeoff power only is required for a very brief portion of the total flight.  Once cruising in the sky overhead, an aircraft doesn’t need as much to maintain altitude. So why not source the energy required at takeoff from an innovation installed on the ground?  Can the burden (and weight) be removed from the aircraft itself?

An assisted takeoff – using some form of propelled acceleration – would mean aircraft could be lighter, with smaller engines consuming less fuel.

So there would be saving in fuel, additional to the fuel saved for take-off.

For a "go around" or a "wave off" are you sure you want a weaker engine?  As your (prospective) passenger, I surely don't.

@KorvinStarmast It's the manufacturer of one of the most succesful airliners publishing this. They consider all factors, including the two you mention, in selecting the power for the engine.

Keep in mind that unlike military aircraft, civilian aircraft are designed for comfort and economy.
Getting any kind of typical airliner retrofitted for catapult use would mean reinforcing the aircraft frame and structure (thus, possibly increasing weight); reinforced or otherwise strengthened wings and engine mounts, possible modifications to the wheels and landing gear -- all this adds weight, which means cost as more fuel would be required (or, less passengers could be carried) thus eating away at any possible savings.
Lets not forget having the runways modified (further costs) and the inevitable delays from runway closures, and the additional delays as the catapult mechanism has to be "reset" after each takeoff.
Not to mention that passengers don't really like the fact that they are jolted about during mild turbulence - just imagine how popular you'll be if you are going to shoot them off like a slingshot.

This is a major point: naval aircraft have a much heavier structure than their land-based counterparts. An F-14 weighs 7 tons more (empty) than an F-15, although it has similar size and only slightly greater payload.

We can't make it since there is 0 commercial aircraft designed with catapults in mind.
Your assumption that it would lead to cost saving is wrong, on many levels. The main one would be that:
Takeoff roll (the part where catapults can act) lasts only a handful of seconds.
Moreover:

You need full power for climb, so you can't step down on the gaz
You can't accelerate much faster due to stress to the airframe and passengers

The catapult would help you accelerating from 0 to V1, but they can't do anything to help you climb to cruise level.
Adding a new system would be very costy, impractical, and only save you a couple of seconds of acceleration.

Also no way to abort takeoff if something goes wrong with aircraft or cable

@TomMcW of course you have the arresting hook at the end of the runaway :p

@TomMcW You could design a catapult such that you can abort the catapult launch at the same speed in the takeoff roll that you can with a standard takeoff - by commanding a stop to the catapult, which, with active braking, could respond faster and with more retarding force than spooling down the jet engines.

This answer ignored the weight penalty of making the airliner strong enough to handle this launch over a long period of time.  (It is otherwise sensible)

If there really was a net economic or safety benefit, they would already be in use. Catapults have been around long enough to be a proven technology where used.
Consider that some airlines have altered their paint schemes because it would shave enough weight from the aircraft to save a meaningful amount of fuel, or add extra payload capacity. If they have looked into and implemented things like that, I'm sure catapults would not have gone overlooked for this long.

The core of it comes down to this for me.
It takes x energy to get from 0 to climb speed. If that energy comes from "the ground" via a catapult or from the engines, there is no real savings. You still have to spend the energy. The only savings may be in cost of fuel to generate that energy. However, even if the savings in fuel costs are extreme, the total energy spent getting to a cruising altitude is minor compared to the energy spent keeping an aircraft up there. The cost of maintaining the catapult, would likely out weigh the cost of the "extra" fuel needed to have the engines produce the climbing energy.
Remember catapults on air craft carriers are not used because the cost savings, there used because there is no other way to get an aircraft up to speed on that short of runway. As VTOL becomes more popular, the catapults are used less and less.
Many Navies have gone to STOBAR or STOVL systems entirely. The current trends seem to be "find me something besides a catapult to get this thing in the air", even when that is at the cost of flexibility.
So to run down your points:

Less Runway : Nope, still gotta land, and even if this were true, very few locations are so tight that runways can't be extended. It may be expensive to do so, but hell, Japan (I think) build a whole new island to hold their airport.
Fuel Savings : Maybe. If you use a steam catapult and make the steam with coal, and the cost difference per unit of work between coal and jet fuel was enough, then yes, there could be a fuel savings. However it would almost certainly get offset by maintenance costs.
Faster turn around : No! It takes time to charge that catapult. It's not instant. You can't just launch a second aircraft as soon as the first one clears the runway. The catapult has to be adjusted, charged, and then fired. In military operations, you can only launch X number of craft. So it's possible that a military catapult could be charged for the entire launch. An airport, however is continuous. So there will be time when charging is needed. There are ways around this, like "side loading" from two sources, so that as one is depleted, the other is charging. But this would increase cost and complexity even more.

Just for the record you can actually launch as many aircraft as the runway is long. One right after the other. I am ignoring all the rules about minimum spacing and such.

VTOL looks nice, but it's not necessarily all that.  The F-35B is still not 100% working.  VTOL also has the disadvantage that the jet exhaust is hitting the deck directly, and is hot enough to melt steel (and runways, which is an issue for the USMC who intend to operate them ashore).  The UK currently has two very expensive aircraft carriers but no aircraft for them, and the USMC have limited air support, because they chose to retire their Harriers before the F-35B was working.

@Graham, yep, that's all true, but the trend is still away from the catapult. So much so, as you point out that the British navy doesn't seem to have anything to use on it's newest carrier. Though I'm pretty sure navies have STOBAR working.

"It takes x energy to get from 0 to climb speed. If that energy comes from "the ground" via a catapult or from the engines, there is no real savings." As a ground propulsion system jet engines are horrifically inefficient. Far more energy goes into the air pushed backwards than into the plane pushed forwards.

Don't forget VTOL requires a great deal more thrust than traditional takeoff.  Horizontal takeoff allows the craft to accelerate along the runway even if thrust is less than gravity, since it is directed perpendicular to gravity.  Therefore it can accelerate until wing lift overcomes gravity. VTOL on the other hand must have enough thrust to overcome gravity.  Bottom line, horizontal takeoff only requires enough thrust to overcome rolling/air resistance(and enough runway to reach takeoff speed), but VTOL requires enough thrust to overcome gravity which is much greater.

@coteyr It may be less a trend than an article of faith from people who don't actually have to do the warfighting.  The US Navy have consciously chosen to stick with catapults.  The British Navy were actually offered electric catapult technology for their carriers by the US, but chose not to take it.

@Graham IIRC the F-35B isn't VTOL, it's STOVL (Short Take-Off Vertical Landing).  The Harriers did much the same; used a catapult (or a runway) for a (short) conventional takeoff but they had to burn fuel / expend munitions / lose significant weight before they could attempt a vertical landing.  Yes, the Harrier COULD do VTOL, but only if they were lightly loaded.  I expect the same is true for the F-35B.

So far all the answers seem to have focused on a short launch, flat catapult, much like you would see on the deck of an aircraft carrier. This does have flaws as listed by many answers.
However, what if it were a ramp? Could be a standard, slightly elevated flat surface or it could be a curved ramp that changes angle exponentially.
Let's say we are retro-fitting an existing airport. That gives us up to one mile of horizontal we could use for our catapult run.
Advantages:

Can accelerate more slowly but over a longer period, allowing a
smoother ride for both the aircraft and the passengers.
Depending on whether it is a flat or curved ramp, can launch up to a few hundred feet into the sky with a decent AOA.
Launch speeds can be near the aircraft's maximum speed rather than just over what is needed to get it into the air, saving on fuel as it is easier to maintain a speed than to accelerate up to one.
Landings can only be in one direction but with gravity to assist breaking, allows for greater deceleration and hopefully decreases time from runway to terminal.
Less likely to clip the tail as you land as the ramp's angle will make the tail proportionately higher.

Disadvantages:

Your aircraft might fall off/get blown off the side of the ramp during landing/taking off.
Non-catapult converted aircraft might not be able to take off from your airfield once they had landed (depends on ramp and aircraft type as well as prevailing winds).
Mentioned earlier but landings pretty much end up as a mono-directional affair, so if the wind is against you, you will have to redirect.

Mitigation:

Build the ramp near the existing runway but not on top of it. As the end of the ramp is pretty high, other structures such as hangars could be incorporated under it. This would save on space and allow regular/unassisted aircraft take-offs and landings on the regular runway but might have serious repercussions should an aircraft fall off the side.
Given the length we have for the ramp, we could use some kind of high speed tractor or pushback instead of a dedicated catapult which would reduce reload/calibration times - same vehicle tows the aircraft to the runway then drives very quickly with it (ok, some handwaving going on here but multiple roving "catapult units" is an idea only needing a solution).


text:
Is a "breakover feature" required for airliner seats?
A "breakover feature" in airline seats is supposed to  assist in absorbing head impacts during a crash, when the passengers are in the brace position. However in some scenarios, most notably the forced water landing of  US Airways Flight 1549, the NTSB investigation found that the seats did not have this feature, which led to shoulder injuries in some of the passengers who were in the brace position.
My questions are:

Is it mandated by the FAA that seats in all commercial airliners
should have a breakover feature?
As a passenger, how do I know if the seats in my plane have this feature?
If the seats don't have such a feature, is it still wise to adopt the brace position in case of a crash or forced landing?

If it were mandated by the FAA, US Airways would have been fined for not having those seats. As a passenger I guess you might not know unless you ask or inspect the seat back assembly. The other way to know is after a crash. If the seats don't have the feature yes it is wise to still use the brace position, that position was developed many years ago and is still in use for a good reason.

Related (regulation at least): Are the current seat belts used in commercial airlines safe enough?

I can't speak to the regs, but my understanding of the brace was primarily to protect your head, not being as exposed to any debris, and not having very far to travel to bang against something.
As far as I understand (and I'm less sure of this), your back being in a curved position like this is better able to absorb impact/jarring than if it's vertical and you drop suddenly, or are whipped forward and backward by suddenly arresting horizontal motion.

authors: Валерий Рожков


text:
What does an "X" at only one end of the runway mean?
I came across aerial imagery of a dirt strip in northern Idaho with a white "X" at one end of the runway, but no markings at the other.  Does this mean the runway is only closed in one direction, or that it's fully closed and one of the markings is missing?
Google Maps image: https://www.google.com/maps/@46.0062134,-114.8435236,590m/data=!3m1!1e3!5m1!1e1

There is a good example here of a small private paved strip marked with an X at 33 15 39 N 117 05 33 W.

I think the X does mean that it is closed in that one direction.  If you look at the 3D view you will see that there is a mountain off the approach end of the X runway making landing on that runway virtually impossible.  If you were to depart on the other non-X runway you would have to turn relatively quickly to avoid hitting the mountain.

Since it's not a paved runway, the usual runway identifier numbers (i.e. 10-28, etc) aren't requited, but the "X" warns off anybody who might attempt an approach to the south on that runway. Good find on the cliff just north of the "X".

Also worth mentioning that this also applies to taxiways and other places aircraft used to go but should now keep away from.

The X closes that approach end.  The other end of the runway is considered usable.  Also, it may be nonstandard, but I have seen X used with a new threshold for a temporary displaced threshold.